I've never found accessory viewfinders all too practical - especially for today's intended purpose of rangefinders which is mostly street photography. Unless the lens being used is completely and absolutely appallingly outside the area of the camera's viewfinder, I don't see the practicallity of an accessory vf. Having to jump back and forth between main vf and accessory vf seems like to me like a possible cause for many missed shots. However, to each his own